This invention relates generally to radio frequency circuits and more particularly to radio frequency switch circuits.
As is known in the art, a radio frequency (RF) switch circuit (hereinafter switch or switch circuit) is a device used to connect and disconnect RF signal paths in an electrical circuit. When connecting RF signal paths, a switch generally provides a bi-directional RF signal path. Thus, signals fed to an input port of the switch appear at an output port of the switch and vice-versa.
When the switch provides a signal path having a relatively low insertion loss characteristic between the input and the output port, the switch is generally referred to as being in the "on" position. When the switch provides a signal path having a relatively high insertion loss characteristic between the input port and the output port the switch is generally referred to as being in the "off" position.
The electrical characteristics of an RF switch include isolation, insertion loss, switching speed and RF power handling capability. Depending upon the particular application it is often necessary to optimize one of these electrical characteristics by trading off the performance of the other electrical characteristics of the switch.
For example, in a pulse radar systems which uses a common antenna to both transmit and receive signals, a component such as a duplexer, a circulator or the like provides isolated signal paths which couple a transmitter and a receiver to the common antenna. Components such as duplexers, circulators or the like, however, have a finite isolation characteristic. During transmit, portions of the transmit signal may leak back to the receiver due to the relatively poor isolation characteristics of the components which provide the receive path and transmit path to the common antenna. Moreover impedance mismatches between the antenna input port and the transmitter may cause high power RF signals from the transmitter to be reflected to the RF receiver. Thus, to protect the receiver from such undesired signals which are provided during the transmit mode, an RF switch circuit capable of withstanding high power RF signal levels may be disposed between the RF receiver and the duplexer for example.
Furthermore, in the pulse radar system the RF switch circuit must be able to switch between its "on" and "off" states at a rate greater than the pulse repetition frequency of the transmitter. When the transmitter provides a signal pulse, the switch is in its "off" or protection state and thus the switch protects the components of the receiver from high power RF signals by disconnecting the RF signal path to the RF receiver. When the transmitter is not providing a signal pulse the switch is in its "on" or "non-protection" state and thus the switch couples RF signals from the duplexer to the receiver.
One type of switch circuit which protects receivers from high power RF signals includes a plurality of PIN diodes connected in shunt across a transmission line at points on the transmission line separated from one another by one quarter wavelength at a particular frequency of operation. Since the diode connected closest to the input port of the switch has the highest power level incident thereon, this diode should have a higher breakdown voltage characteristic than the diode connected closest to the output port of the switch. Thus, the breakdown voltage of each PIN diode should correspondingly decrease from the input port to the output port of the switch.
The increase in breakdown voltage is typically achieved by increasing the thickness of the intrinsic region of the PIN diode. As is known, as the intrinsic region thickness increases the shunt resistance of the diode increases. However, as is also known, as the intrinsic region thickness is increased the capacitance of the diode decreases. This results in a concomitant decrease in the switching speed of the diode (i.e. its takes longer to switch the diode between its conducting and nonconducting states). Thus a trade off is made between the power handling capability and switching speed of the switch.
When the switch is in the non-protection mode the diodes are reversed bias and provide high shunt resistance so that substantially all of the RF signal power fed to the input port of the switch propagates along the RF transmission line relatively unattenuated to the output port of the switch.
However when the switch is in its protection mode the diodes are placed in their forward conducting state and provide a low impedance path between the RF transmission line and ground. RF signals fed to the input port of the switch are shunted to ground through the forward biased diodes. A portion of the RF power dissipates in the diodes due to the resistance of the diodes and thus the diodes are heated.
If the diodes are unable to dissipate the heat which is generated the diodes are damaged. Thus it is desirable to channel the heat away from the PIN diodes via a heat sink.
Conventional waveguide switch circuits include a plurality of diodes disposed in a transverse plane of the waveguide transmission line. By disposing the diodes in the transverse plane of the waveguide transmission line a problem is that the diodes are not provided with a heat sink. Thus, such a waveguide switch circuit is not able to handle high power RF signals.
Alternatively, in another approach a diode may be disposed in a bottom wall of a waveguide transmission line having a rectangular cross-section and partially protruding into the waveguide transmission line with an electrically conductive post disposed between the diode and a top wall of the waveguide transmission line. By disposing the diode partially in the waveguide wall and partially in the waveguide transmission line a heat sink is provided to the diode. However, one problem with this technique is that such circuits are relatively difficult to tune. Nevertheless, this technique is used in waveguide switch circuits because of the low insertion loss characteristic the switch provides in its "on state. Thus, when the diode is forward biased placing the switch in its "on" state the structure is highly resonant and therefore provides the switch having a relatively low insertion loss characteristic.
Other microwave circuits also use resonant structures disposed along the waveguide transmission line. For example, as is known in the art, a negative resistance device, such as an IMPATT diode for example, is often used as an oscillator or an amplifier to convert DC power to RF power. IMPATT diode oscillator circuits generally include a waveguide transmission line having a rectangular cross-section and having an electrically conductive member transversely disposed across one end of the waveguide to provide a short circuit impedance characteristic across the end of the waveguide transmission line. Thus the waveguide transmission line having a short circuit at one end provides a resonant waveguide cavity.
A plurality of IMPATT diodes which feed RF signals into the cavity may be disposed in the side walls of the waveguide cavity and spaced along the sidewalls of the cavity at half wavelength intervals. Each of the IMPATT diodes are provided having matching circuits to match the impedance of the IMPATT diodes to the impedance of the waveguide cavity over a pre-determined range of frequencies.
In one embodiment of an IMPATT diode oscillator circuit described in U.S. Pat. No. 4,583,058 and assigned to the assignee of the present invention each one of the diode oscillators includes a center conductor having a first end connected to a first end of an IMPATT diode. A second end of the IMPATT diode is connected to a heat sink. A ring shaped member is disposed about the center conductor and spaces the IMPATT diode from the resonant waveguide cavity. A second end of the center conductor has a tapered sleeve section to provide a stabilizing load used to terminate the diode oscillator in a characteristic impedance. Several of such IMPATT diode oscillator circuits may be disposed around a power combiner circuit which combines the power of such IMPATT diode oscillator circuits to provide high power RF signals over a relatively broad range of operating frequencies.